The present invention relates to a process for the gas-phase (co-)polymerisation of olefins, more particularly in a fluidised bed reactor.
Processes for the co-polymerisation of olefins in the gas phase are well known in the art. Such processes can be conducted for example by introducing the gaseous monomer and comonomer into a stirred and/or gas fluidised bed comprising polyolefin and a catalyst for the polymerisation.
In the gas fluidised bed polymerisation of olefins, the polymerisation is conducted in a fluidised bed reactor wherein a bed of polymer particles is maintained in a fluidised state by means of an ascending gas stream comprising the gaseous reaction monomer. The start-up of such a polymerisation generally employs a bed of polymer particles similar to the polymer which it is desired to manufacture. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of the monomer, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favoured process employs a fluidisation grid to distribute the fluidising gas to the bed, and to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidisation grid. The fluidised bed consists in a bed of growing polymer particles. This bed is maintained in a fluidised condition by the continuous upward flow from the base of the reactor of a fluidising gas.
The polymerisation of olefins is an exothermic reaction and it is therefore necessary to provide means to cool the bed to remove the heat of polymerisation. In the absence of such cooling the bed would increase in temperature and, for example, the catalyst becomes inactive or the bed starts to fuse. In the fluidised bed polymerisation of olefins, the preferred method for removing the heat of polymerisation is by supplying to the polymerisation reactor a gas, the fluidising gas, which is at a temperature lower than the desired polymerisation temperature, passing the gas through the fluidised bed to conduct away the heat of polymerisation, removing the gas from the reactor and cooling it by passage through an external heat exchanger, and recycling it to the bed. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidised bed at the desired polymerisation temperature. In this method of polymerising alpha olefins, the recycle gas generally comprises the monomer and comonomer olefins, optionally together with, for example, an inert diluent gas such as nitrogen or a gaseous chain transfer agent such as hydrogen. Thus, the recycle gas serves to supply the monomer to the bed, to fluidise the bed, and to maintain the bed at the desired temperature. Monomers consumed by the polymerisation reaction are normally replaced by adding make up gas or liquid to the polymerisation zone or reaction loop.
Amongst the catalysts known for polymerisation or copolymerisation of olefins are late transition metal catalyst the s. Late transition metal complexes such as those disclosed in WO 96/23010 or WO 99/12981 are particularly effective. By “late transition metal” is meant a metal from Groups VIIIb or Ib (Groups 8-11) of the Periodic Table. In particular the metals Fe, Co, Ni, Ru, Rb, Pd, Os, Ir, and Pt are preferred, especially Fe, Co and Ni.
However with all catalysts there is always the potential for fouling in gas phase polymerisation processes. Non uniform fluidisation, poor heat transfer in the polymerisation process and activity kinetic profile can all contribute to such problems. Such fouling often manifests itself in the form of “cold bands” in the temperature profile of the reactor, showing areas where the polymer has formed a film on the reactor surface. Elimination of such cold bands is therefore desirable. There is thus a need in the art to find a process for producing polyolefins on gas phase industrial plants using late transition metal catalysts in which the degree of fouling is reduced or eliminated.
Our own copending unpublished application WO GB00/01690 discloses that a process aid additive selected from at least one of
(1) a polysulfone copolymer,
(2) a polymeric polyamine, and
(3) an oil-soluble sulfonic acid
can alleviate the above-mentioned problems.